1. Field of the Invention
The present invention relates to a digital radio receiving apparatus used in a radio communication system for transferring digital data, more particularly, a digital radio receiving apparatus having of a transversal demodulator unit included therein.
2. Description of the Related Art
A digital radio communication system can be operated under a variety of known methods. Among the usual methods, a phase shift keying (PSK) method and quadrature amplitude modulation (QAM) method are suitable for transferring large amounts of data within a limited frequency band. A digital radio communication system operated under the PSK or QAM method, however, often is subject to transmission distortion in the transmission band due to fading and waveform deterioration due to inter symbol interference.
In order to deal with this distortion and deterioration, the digital radio receiving apparatus is usually equipped with a demodulator unit able to equalize a received signal in which transmission distortion and waveform deterioration occur in an intermediate frequency (IF) band or the baseband. This demodulator unit, known as a transversal demodulator unit, includes therein a transversal equalizer and a carrier recovery circuit. In the unit, a carrier wave is reproduced from the received signal. Then the received signal is synchronously detected using the thus reproduced carrier wave, and an original data signal is obtained through level discriminations of the thus synchronously detected signal.
A digital radio receiving apparatus having a transversal demodulator often does not accurately reproduce the data due to certain types of signal disturbance. Specifically, there is no difficulty reproducing data when the carrier recovery circuit is first set in the normal operation state, i.e., synchronous detection is normally performed, and then the transversal equalizer starts its equalization operation. The carrier recovery circuit can be maintained in its normal operation state even if the distortions of the received signal increase to a considerably high level, since the so-called eye pattern can be opened, to match increases in the distortion level, by operation of the transversal equalizer. Contrary to the above, there is difficulty reproducing the data when a received signal having a considerably larger amount of distortion, is received by the demodulator unit before the synchronous detection by the carrier recovery circuit is complete. That is, the carrier recovery circuit can not reach its normal operation state, since, under this circumstance, the so-called tap coefficients of the transversal equalizer deviate from their optimum values by a large amount.
In order to eliminate this trouble, there is known the prior art a technique using a so-called "training pattern". A training pattern, however, is not very practical in actual use with a radio transmission line. The training pattern is useful for a wire transmission line due to the wide frequency spectrum used to complete an initial condition setting. In a radio transmission line, however, such a wide frequency spectrum cannot be used due to inherent conditions of the spatial media through which the radio transmission line is created.